This animation shows elliptical orbits with different eccentricities. It also shows how the Sun is at the focus of an ellipse, and some of the math behind elliptical orbits.
Click on image for full size Original animation by Windows to the Universe staff (Randy Russell).

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Elliptical Orbits

You may think that most objects in space that orbit something else move in
circles, but that isn't the case. Although some objects follow circular orbits,
most orbits are shaped more like "stretched out" circles or ovals. Mathematicians
and astronomers call this oval shape an ellipse. All of the planets in our
Solar System, many satellites, and most moons move along elliptical orbits.

An ellipse can be very long and thin, or it can be quite round - almost like
a circle. Scientists use a special term, "eccentricity", to describe how round
or how "stretched out" an ellipse is. If the eccentricity of an ellipse is
close to one (like 0.8 or 0.9), the ellipse is long and skinny. If the eccentricity
is close to zero, the ellipse is more like a circle.

Earth moves around the Sun in an elliptical orbit. Earth's orbit is almost
a perfect circle; its eccentricity is only 0.0167! Pluto has the least circular
orbit of any of the planets in our Solar System. Pluto's orbit has an eccentricity
of 0.2488.

The Sun isn't quite at the center of a planet's elliptical orbit.
An ellipse has a point a little bit away from the center called the "focus".
The Sun is at the focus of the ellipse. Because the Sun is at the focus,
not the center, of the ellipse, the planet moves closer to and further away
from the Sun every orbit. The close point in each orbit is called perihelion.
The far away point is called aphelion.

Johannes Kepler, a German astronomer
who lived in the early 17th century, discovered some important laws about orbits.
Kepler's First
Law of Planetary Motion states
that planets move in elliptical orbits. His Second
Law explains how planets
move faster when they are close to the Sun (near perihelion) than when they
are far away (near aphelion).

If you want to know more about the mathematics of ellipses, take a look at
the advanced version of this page.

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